It has been appreciated for over 30 years that visual experience during an early postnatal critical period of development produces permanent modifications of the connectivity, physiology and function of the visual cortex. Besides the obvious relevance of this neural plasticity to the development of visual capabilities in humans and animals, it seems likely that similar processes from the basis for some forms of learning and memory in the adult brain. Considerable progress has been made in identifying the specific changes in visual cortex that result from rearing the animals in different environments, but the detailed mechanisms that underlie these modifications have remained elusive. Recently, however, advance in understanding the receptor mechanisms that mediate synaptic excitation and inhibition in the visual cortex during development have provided an exciting new opportunity to investigate the mechanisms of experience- dependent brain modification. The long-term goal of this project is to elucidate these molecular mechanisms of experience-dependent cortical plasticity. It is hypothesized that naturally occurring synaptic enhancements employ the mechanisms that underlie long-term synaptic potentiation (LTP), a type of plasticity that can be elicited in cortical synapses by tetanic electrical stimulation. To examine this hypothesis, a preparation has been introduced in which LTP can be elicited in the geniculo-cortical projection in vivo.. The aims of the research are (1) to characterize the laminar, spatial and temporal characteristics of LTP induced in visual cortex by tetanic stimulation of the lateral geniculate nucleus (LGN); (2) to investigate the consequence of LTP induction in vivo on visually evoked potentials and visual cortical receptive fields in normal and dark-reared (DR) animals, and to compare these with the effects of light exposure in dark-reared animals; (3) to investigate the interaction of naturally occurring synaptic enhancement and LTP in the visual cortex in vivo;; and (4) to document structural changes which follow LTP induction in vivo and compare these with the changes that follow induction of synaptic enhancement caused by light exposure (LE) in dark-reared animals. This experiment promises to establish, for the first time in any light exposure (LE) in dark-reared animals. These experiments promise to establish, for the first time in any system, which LTP and naturally occurring synaptic plasticity utilize common mechanisms.